Effect of aspirin dosage and enteric coating on platelet reactivity

Whole Blood Lumiaggregation: Evaluation of Reagents

We evaluated the whole blood lumiaggregation system, which analyzed the optimal sample dilutions and ago nist concentrations. We also showed that platelet aggregation using the whole blood impedance technique, as compared to the platelet-rich plasma optical method, yielded similar informa tion. In the extension of that study, we further evaluated the stability of the reagents used in platelet aggregation. The most commonly used agonists thrombin, ristocetin, arachidonic acid, adenosine diphosphate, and collagen were monitored over a 1-year period. Throughout the entire period, aliquots of the reconstituted reagents were stored at -20°C, -50°C, and -70°C, with the exception for collagen, which was kept at 4°C. Every 2 weeks tests were performed using the whole blood from the same healthy volunteer. Platelet aggregation and aden osine diphosphate release were measured after stimulation with 1.0 U/mL thrombin, 1.0 mg/mL ristocetin, 0.5 mM arachidonic acid, 10 μM adenosine diphosphate or 3 μg/mL collagen. The results indicated that thrombin was stable at all temperatures over the 1-year period. Platelet agglutination with ristocetin was similar among samples for about 2 months; after that time some deterioration of ristocetin was noticed, especially at -20°C. Reconstituted arachidonic acid, frozen at -20°C, was stable for about 1 month, and at the lower temperatures this agonist was good for 4 months. On the contrary, adenosine diphosphate and collagen exhibited stability throughout the 1- year period. Based on the information provided by this study, we encourage more laboratories to use whole blood lumiaggre gation to evaluate platelet function.

Platelet aggregation responses vary over a period of time in healthy controls

Platelet aggregation study is performed to investigate platelet function abnormality. A normal healthy control sample is usually run with the patient sample as a quality control measure. At our institution, we observed variations in platelet aggregation responses in our normal repeat controls. Therefore, we analysed aggregation parameters in these controls. Whole blood aggregation studies were performed with adenosine diphosphate (ADP), arachidonic acid (AA), collagen and ristocetin. Adenosine triphosphate (ATP) secretion was also measured simultaneously by leuciferin-leuciferase reaction. During a 5-year period, a total of 86 studies were performed on seven controls. Aggregations were within the acceptable range in 67% of the time. Collagen was the most affected agonist in our study. On five occasions, four controls had subnormal aggregations with two agonists. All abnormal responses were hypoaggregation except for two who had hyperaggregation with collagen and AA. Only one out of seven controls was always normal. In the presence of a subnormal control result, a new control was run before releasing the patient's platelet aggregation results. These findings suggest that many physiological factors, other than medications, may affect platelet function even in normal individuals. Therefore, a repeat study at a later date to demonstrate a reproducible abnormality would be prudent before labeling a patient's platelets abnormal.

Clinical bioequivalence of a dose of clopidogrel Leti Cravid tablets 75 mg versus clopidogrel Sanofi Plavix tablets 75 mg administered on a daily dose for 7 days on healthy volunteers: a clinical trial

Patients undergoing percutaneous coronary intervention procedures, as in patients with coronary disease, should receive treatment indefinitely with acetylsalicylic acid and clopidogrel. New brands of clopidogrel have been developed at lower costs, for helping to avoid premature suspension of antiplatelet therapy, as Cravid Leti Laboratories clopidogrel. Its effectiveness and safety must be compared with Plavix international standard. A prospective, comparative, cross-over, and randomized study was conducted in healthy volunteers. Each group received 1 tablet of Clopidogrel Leti or Clopidogrel Sanofi, 75 mg in a single dose daily for 7 days, followed by 7-day washout period before administration of second treatment. Platelet aggregation was measured at the start of each period and at 7 days of treatment through optical aggregometry, using an optical aggregometer 490-2D Chrono-Log, with a self-calibration system working with platelet-rich plasma with readings 0%-100% of light transmission. An important decrease of platelet aggregation was observed in both groups at 7 days of treatment of more than 50%, independent of adenosine diphosphate reactive (Helena and Chrono-Log) used for aggregation (P < 0.05). The relationship between the mean and 90% confidence interval ratio obtained with the 2 different adenosine diphosphate brands were between 80% and 125%, therefore, it can be considered that both brands are bioequivalent and perfectly exchangeable.

Aspirin Use and Post-operative Bleeding from Dental Extractions

Aspirin is a common, chronically administered preventive treatment for cardiovascular disease, but is often discontinued prior to invasive dental procedures because of concern for bleeding complications. We hypothesized that aspirin does not cause increased bleeding following a single tooth extraction. Thirty-six healthy persons requiring a tooth extraction were randomized to receive 325 mg/day aspirin or placebo for 4 days. Cutaneous bleeding time (BT) and platelet aggregation tests were obtained prior to extraction. The primary outcome measure, oral BT, and secondary bleeding outcomes were evaluated during and following extraction. No significant baseline differences, except for diastolic blood pressure, were found between groups. There were no differences in oral BT, cutaneous BT, secondary outcome measures, or compliance. Whole-blood aggregation results were significantly different between the aspirin and placebo groups. These findings suggest that there is no indication to discontinue aspirin for persons requiring single-tooth extraction.

Aspirin resistance and adverse clinical events in patients with coronary artery disease

PURPOSE

We sought to determine the clinical significance of aspirin resistance measured by a point-of-care assay in stable patients with coronary artery disease (CAD).

METHODS

We used the VerifyNow Aspirin (Accumetrics Inc, San Diego, Calif) to determine aspirin responsiveness of 468 stable CAD patients on aspirin 80 to 325 mg daily for > or =4 weeks. Aspirin resistance was defined as an Aspirin Reaction Unit > or =550. The primary outcome was the composite of cardiovascular death, myocardial infarction (MI), unstable angina requiring hospitalization, stroke, and transient ischemic attack.

RESULTS

Aspirin resistance was noted in 128 (27.4%) patients. After a mean follow-up of 379+/-200 days, patients with aspirin resistance were at increased risk of the composite outcome compared to patients who were aspirin-sensitive (15.6% vs 5.3%, hazard ratio [HR] 3.12, 95% confidence intervals [CI], 1.65-5.91, P < .001). Cox proportional hazard regression modeling identified aspirin resistance, diabetes, prior MI, and a low hemoglobin to be independently associated with major adverse long-term outcomes (HR for aspirin resistance 2.46, 95% CI, 1.27-4.76, P = .007).

CONCLUSIONS

Aspirin resistance, defined by an aggregation-based rapid platelet function assay, is associated with an increased risk of adverse clinical outcomes in stable patients with CAD.

Impact of Enteric vs Non-Enteric Coated Aspirin Preparations on Efficacy and Toxicity

This feature examines the impact of pharmacologic interventions on the treatment of the critically ill patient — an area of health care that has become increasingly complex. Recent advances in drug therapy (including evolving and controversial data) for adult intensive-care-unit patients will be reviewed and assessed in terms of clinical, humanistic, and economic outcomes. Direct questions or comments to Gil Fraser, PharmD, at fraseg@mmc.org or Sandra Kane-Gill, PharmD, MSc, at kanesl@upmc.edu .

Effect of increasing doses of aspirin on platelet function as measured by PFA-100 in patients with diabetes

INTRODUCTION

Platelets of diabetic patients have been reported to be less sensitive to aspirin. The aim of this study is to compare a medium (300 mg) and low (100 mg) dose of aspirin on platelet function in diabetic patients.

METHODS

We have included one hundred and two patients with type 2 diabetes mellitus. Platelet function was measured as closure time (CT) with the Platelet Function Analyzer (PFA)-100 before the administration of aspirin. Initially the patients were given 100 mg aspirin once daily for seven days, and then the measurements were repeated. If the CT exceeded the upper limit of 300 s, the study was terminated. If not, the patients continued the aspirin therapy with a dose of 300 mg daily for another seven days, and the CTs were measured again.

RESULTS

After taking 100 mg aspirin, the CT significantly increased from 126+/-29 s to 256+/-66 s (p<0.001). In 68 of 102 (67%) patients, the CT increased to 300 s. In the remaining 34 patients, the baseline CT was 113+/-29, and increased to 170+/-45 s after 100 mg aspirin (p<0.001). In these patients, there was a further increase in the CT from 170+/-45 to 229+/-75 s following 300 mg aspirin (p<0.001). On average, the CT was increased by 60% and 39% following ingestion of 100 and 300 mg aspirin, respectively. CT>300 s were obtained in 15 (44%) of 34 patients after 300 mg aspirin.

CONCLUSIONS

Although, a daily dose of 100 mg aspirin effectively inhibited platelet function in a majority of diabetics, a considerable proportion of patients showed a greater platelet inhibition with the use of 300 mg aspirin. The PFA-100 closure time may be used to separate those patients who require a higher dose of aspirin to achieve desired antiplatelet effect.

Aspirin Resistance in Cardiovascular Disease: A Review

BACKGROUND

Aspirin is effective at reducing the cardiovascular event rate in defined patient groups. The introduction of antiplatelet therapies other than aspirin and the concept of aspirin resistance have led to critical reappraisal of current treatment. This review aims to clarify the evidence for aspirin resistance in patients with atherosclerosis.

METHODS

Medline search was performed to identify publications concerned with antiplatelet effects of aspirin and failure of aspirin therapy. Manual cross referencing was also performed.

RESULTS AND CONCLUSION

Wide variations in the rate of aspirin resistance (5.5-75%) have been reported. The lack of consensus on an appropriate definition and the number of different tests used to investigate aspirin resistance needs to be addressed. There are few studies where the primary aim was to document aspirin resistance or aspirin non-response. Further work should aim to investigate if aspirin resistance is clinically important and, if it is, what treatments may be beneficial to the at risk patient.

Effect of different aspirin doses on platelet aggregation in patients with stable coronary artery disease

BACKGROUND

Aspirin is widely used as an antiplatelet agent in the primary and secondary prevention of cardiovascular disease. In order to spare prostacyclin formation and reduce gastrointestinal side-effects, very low doses of aspirin have been introduced. However, it remains unclear whether these low doses are equally effective with respect to inhibition of platelet aggregation.

AIMS

In a randomized, controlled study in 60 patients with stable coronary artery disease, the effects on platelet aggregation of five doses (50, 80, 100, 162.5 and 325 mg) of aspirin, which are widely used in clinical practice, given for 70 days, were investigated. Two reagents, adenosine diphosphate (ADP) and epinephrine, were used to induce platelet aggregation in platelet-rich plasma. An age- and sex-matched group of people without coronary artery disease served as the control.

RESULTS

ADP- and epinephrine-induced platelet aggregation was 78.2 +/- 12.8% and 76.7 +/- 15.5% of maximum aggregation in the control group. Aspirin inhibited platelet aggregation in a dose-dependent manner. Minimum platelet aggregation was observed at a dose of 325 mg aspirin (27.5 +/- 17.4% with ADP). Doses of 50 and 80 mg aspirin were much less effective in inhibiting platelet aggregation (59.1 +/- 11.4% and 50.3 +/- 12.1% with ADP, respectively). Doses of 100 and 162.5 mg aspirin produced significantly greater inhibition of platelet aggregation than lower doses (36.2 +/- 11.7% and 38.5 +/- 19.8% platelet aggregation with ADP, respectively).

CONCLUSION

Our results demonstrate that doses of aspirin less than 100 mg are not as effective at inhibiting platelet aggregation as doses greater than 100 mg.

Supravalvular mitral stenosis in a cat

A 3-year-old, 4-kg, castrated male domestic shorthair cat presented with signs of progressive respiratory distress. Thoracic radiographs showed pulmonary edema and pleural effusion. Echocardiography revealed a perforate membrane immediately above the mitral valve that divided the left atrium into proximal and distal chambers. The left auricle was proximal to the dividing membrane and connected to the markedly enlarged proximal left atrial chamber, consistent with the diagnosis of supravalvular mitral stenosis (SMS). Position of the obstructing membrane relative to the left auricle distinguishes SMS from cor triatriatum sinister (CTS). In CTS, the left auricle is distal to the dividing membrane and connects to the distal left atrial chamber.

Aspirin in the prophylaxis of coronary artery disease

Aspirin has been used for more than 100 years, but its mechanisms of action have only been understood in the past 20 years. Aspirin interferes with arachidonic acid metabolism in platelets and endothelial cells and thereby reduces thromboxane A2 and prostacyclin. It also has other mechanisms of action, including anti-inflammatory roles, protection from oxidative stress, enhancement of fibrinolysis, and suppression of plasma coagulation and platelet-dependent inhibition of thrombin generation. It has been used for primary and secondary prevention of myocardial ischemia, and for primary and secondary prevention of cerebrovascular ischemia. We review the 5 pivotal studies relating to primary prevention for cardiovascular risk and the many studies relating to secondary prevention of myocardial ischemia. We also review the utility of aspirin in primary prevention of myocardial infarction and stroke. We conclude that aspirin is one of the most potent drugs ever discovered and that its effects extend well beyond those of cycloxoxygenase enzyme inhibition. Aspirin treatment does not preclude control of underlying and comorbid conditions such as diabetes mellitus, hypertension, and dyslipidemia. For most patients, a daily dose of 325 mg is optimal. Patients must understand the potential for gastrointestinal upset and hemorrhagic complications. The utility of aspirin is greater in coronary artery disease prevention than in cerebrovascular prevention.

A new method for measuring inhibition of platelet function by nonsteroidal antiinflammatory drugs

Aspirin is widely used to help prevent vascular occlusion caused by atherosclerotic vascular disease. We used a platelet-aggregation assay (PAA) to evaluate the reliability of a proprietary platelet agonist, platelet prostaglandin agonist (PPA), to detect the amount of platelet inhibition induced by four different classes of nonsteroidal antiinflammatory drugs (NSAIDs) with antiplatelet effects. Twenty normal donors were evaluated before and 24 hours after ingestion of 325 mg of aspirin. With 125 micromol/L PPA, the slope of the PPA-PAA curve completely differentiated aspirin-treated from normal platelets. The aspirin platelet slope, 27.9 +/- 2.0 (range 5.5-47), was significantly decreased (P <.001) compared with the findings before administration of aspirin, 75 +/- 3.1 (range 50-125). Additionally, the time elapsed before 50% platelet aggregation (T(50)) with aspirin, 10.1 +/- 0.7 minutes (range 4.7-17), was significantly prolonged (P <.05) compared with the mean time before administration of aspirin (4.2 +/- 0.2 minutes, range 1.7-6.4). Aspirin in a daily dosage of 325 mg for 14 days produced significantly greater inhibition of PPA-PAA than that induced by a single 325-mg dose (P <.001). The long-term platelet-inhibitory effects of aspirin in 9 normal volunteers were evaluated with PPA-PAA 2, 8, 24, 48, 72, and 96 hours after a single dose of aspirin, 81 or 325 mg. Compared with the preaspirin slope, 79.6 +/- 1.9, the maximal decrease in slope occurred after 2 hours for both 81 mg (61.3 +/- 6.7) and 325 mg (12.1 +/- 1.8). The decreased slopes and increased T(50) observed at 2, 8, and 24 hours (P <.001) reflected the greater degree of platelet inhibition with 325 mg than with 81 mg aspirin. Inhibition of PPA-PAA was observed with nonaspirin nonsteroidal antiinflammatory drugs (NNSAIDs), but, compared with aspirin, the inhibition was minimal. PPA-PAA may be used to help measure the magnitude of NSAID-induced inhibition of platelets.

Effect of short-term aspirin use on C-reactive protein

Markers of inflammation, such as C-reactive protein (CRP) and fibrinogen, have been shown to be predictive of cardiovascular disease. In the Physicians Health Study, the magnitude of reduction in the risk of myocardial infarction with aspirin therapy was related to baseline CRP levels, raising the possibility that the protective effect of aspirin may be due to antiinflammatory properties in addition to its antiplatelet effect. We therefore investigated whether aspirin therapy lowers CRP levels. Because heavy physical exertion is a well-known trigger of myocardial infarction, we also investigated the effect of aspirin on CRP levels before and after strenuous exercise. Thirty-two healthy men, aged 29 +/- 6 years, were enrolled in a randomized, double-blind, parallel study. Blood samples were obtained immediately before and after maximal treadmill exercise at baseline and following 7 days of aspirin therapy (81 or 325 mg). The levels of CRP, as measured by ELISA, increased by 13% following exercise (P < 0.0001). However, aspirin did not significantly alter CRP levels, either at rest (0.81 +/- 0.13 mg/L before aspirin vs. 0.78 +/- 0.13 mg/L on aspirin) or following exercise (0.92 +/- 0.13 mg/L before aspirin vs. 0.86 +/- 0. 13 mg/L on aspirin), P = 0.73. When the resting and postexercise data were combined, the levels were 0.87 +/- 0.13 mg/L before aspirin and 0.82 +/- 0.13 mg/L on aspirin (a nonsignificant 6% reduction, P = 0.20). In conclusion, in healthy male subjects CRP levels were not significantly reduced by short-term aspirin therapy. Our data, taking together with other reports, suggest that aspirin may not affect the levels of inflammatory markers. However, further studies are needed with a longer duration of therapy, among subjects with coronary heart disease, and using additional markers of inflammation besides CRP to determine the long-term effects of aspirin use.

Upright posture and maximal exercise increase platelet aggregability and prostacyclin production in healthy male subjects

BACKGROUND

It is well accepted that heavy physical exertion can trigger the onset of myocardial infarction, but the mechanism is uncertain. As platelet and endothelial function play an important role in thrombotic events, platelet and prostacyclin responses to maximal treadmill exercise were studied.

METHODS/RESULTS

The study subjects were 40 healthy men, mean (SEM) age 29 (5) years. Platelet aggregation was measured on a four channel aggregometer. Plasma 6-keto-prostaglandin F1alpha was analysed using an enzyme immunoassay technique. Upright posture and exercise produced an increase in platelet aggregability, as indicated by a fall in the threshold concentration of adrenaline (epinephrine) from 7.6 (1.5) microM at rest to 4.3 (1.0) microM after exercise (p = 0.002). The collagen lag time became significantly shorter with exercise (from 79.1 (3.1) seconds at rest to 71.9 (2.6) seconds after exercise, p = 0.003). Exercise was also associated with a 55% increase in plasma 6-keto-prostaglandin F1alpha (from 38.1 (75%CI 29.0 to 46.5) pg/ml at rest to 59.2 (47.3 to 66.8) pg/ml after exercise, p<0.001).

CONCLUSIONS

In healthy male subjects, upright posture and maximal exercise increased platelet aggregability but this increase was counteracted by an increase in prostacyclin production. In patients with endothelial dysfunction, a reduced prostacyclin response to exercise may promote a transient prothrombotic imbalance that may trigger cardiovascular disease onset.

Comparison of enteric-coated aspirin and uncoated aspirin effect on bleeding time

Aspirin therapy is an essential part of the drug regimen for patients with acute myocardial infarction (MI), unstable angina, or after coronary angioplasty and coronary stenting. Recognizing this importance, this study sought to compare the bleeding time in two groups of 10 normal volunteers 4 hr after ingestion of either an enteric-coated aspirin or an uncoated aspirin, assuming that a difference between the two groups could be clinically significant. Defining < or = 8 min as normal, 80% of the uncoated group developed abnormal bleeding times, compared to 10% of the enteric-coated group (P < 0.01). The study demonstrates a significant difference between the two types of aspirin preparations on bleeding times in normal individuals. This strongly suggests that some enteric-coated aspirin preparations may not be as effective as uncoated aspirin in acutely decreasing platelet aggregation. Therefore, uncoated aspirin is recommended in the setting of acute MI, unstable angina, or after percutaneous transluminal coronary angioplasty.